1. Field of the Invention
The present invention is directed to a computed tomography system of the type having a gantry rotating around a patient acceptance space in a stationary gantry housing, the gantry carrying an x-ray source connected to a cooling device.
2. Description of the Prior Art
99% of the energy utilized in generating x-rays is converted into heat, the elimination thereof presenting rather substantial problems particularly in modem examination installations such as computed tomography or angiography systems wherein extremely high powers are used. A particular problem arises in computed tomography from the fact that the x-ray source is located on a continuously rotating gantry, and the rotational axis is not accessible for rotary bushings because of the necessary clearance for pushing the patient through. Although the electrical energy can be easily supplied via wiper rings, the waste heat, however, cannot be eliminated by direct cooling with fluid during the rotary motion.
Heretofore, rotating anode x-ray tubes have been generally employed in computed tomography systems, which intermediately store the arising heat in the anode dish and permanently emit it mainly via thermal radiation (convection) to the surrounding cooling and insulating oil. After reaching the anode limit temperature, long compulsory pauses must always take place in order to be able to adequately cool the anode dish by radiation via the relatively poor means of thermal radiation. The oil circulates in a closed circulation loop through a co-rotating heat exchanger that outputs the heat to the air in the gantry housing. Another stationary heat exchanger in the gantry housing cools the heated gantry air and eliminates the heat to, for example, a permanently installed liquid coolant system. Given this known structure, two high thermal resistances necessarily lie in series: the primarily radiant transport from the rotating anode through the tube vacuum into the cooling oil, and the two heat exchangers that decouple the gantry rotation.
Due to the T4 law the radiation transport is in fact rather efficient at high dish temperatures (around 2000xc2x0 C.), but decreases greatly given dropping temperature and the existing heat capacity of the dish is not completely utilized for time reasons. The two series oil/air and air/water heat exchangers must be rather bulky because of the low heat capacity of the air and because of only a slight, allowable temperature increase of the air.
U.S. Pat. No. 4,115,697 discloses a computed tomography system wherein a circulation loop for a liquid coolant surrounding the x-ray source and having a circulating pump, a radiator and a reservoir is provided on the gantry. In standstill periods of the gantry, the radiator is arranged in the airstream of a blower.
U.S. Pat. No. 5,610,968 discloses a computed tomography system having an x-ray source connected via a first heat exchanger, operated at the primary and secondary sides with liquid, to the primary side of a second heat exchanger, operated with liquid at the primary side and with air at the secondary side. The second heat exchanger is active during standstill periods of the gantry in order to cool the liquid flowing in a circulation loop including the secondary side of the first heat exchanger and the primary side of the second heat exchanger, cooling taking place via the secondary side of the second heat exchanger.
German OS 197 48 281 is directed to a cooling device for employment with an x-ray source in a computed tomography system, that has a heat exchanger attached on the gantry that is traversed at the primary side by a liquid coolant in thermal contact with the x-ray source and operating with air at the secondary side.
An object of the present invention is to provide a computed tomography system of the type initially described having improved cooling achieved with only short standstill times after an examination, in order to return the system to the desired initial temperatures.
A further object of the invention is to provide a computed tomography system of the type initially described wherein effective cooling of the x-ray radiator and the x-ray tube contained therein, is assured with a small installation space.
These objects are inventively achieved in a computed tomography system having a gantry rotatable around a patient acceptance space in a stationary gantry housing, the gantry carrying an x-ray radiator containing an x-ray tube in thermal contact with a liquid coolant, the x-ray radiator being connected to a heat exchanger that is attached to the gantry and which has a primary side and a secondary side. Liquid coolant flows In a primary circulation loop that includes the primary side of the heat exchanger and a coolant pump pumps the liquid coolant through the primary circulation loop in standstill periods of the gantry for cooling. A liquid coolant flows through the secondary side of the heat exchanger, this liquid coolant being supplied to the secondary side in standstill periods of the gantry from a source that is stationary with respect to the gantry and being supplied from the secondary side to a sink that is stationary with respect to the gantry.
An important feature of the invention is that the heat exchanger attached to the gantry, and therefore rotating with the gantry during the implementation of an examination is not constantly active, which would make it necessary to employ air as the coolant at the secondary side. Instead, the heat exchanger is only placed into operation during standstill periods of the gantry, so that a liquid coolant, preferably water, can be employed at the secondary side of the heat exchanger, since this is supplied to the secondary side of the heat exchanger from a source that is stationary relative to the gantry and flows from the secondary side of the heat exchanger to a sink that is stationary relative to the gantry. The source and the sink for the liquid coolant thus can be components of a circulation loop for the coolant which also has a cooling device for the liquid coolant that is stationary relative to the gantry. As a result of employing a liquid coolant at the secondary side of the heat exchanger, the heat exchanger exhibits only slight dimensions compared to a heat exchanger that operates with liquid at the primary side and with air at the secondary side. Moreover, the thermal resistance to be overcome in a heat exchanger operating with liquid both at the primary side and at the secondary side is lower, so that a more effective cooling of the x-ray radiator, or of the x-ray tube, is assured be the inventive computed tomography system.
In a preferred embodiment of the Invention, an x-ray radiator Is used that contains a rotating bulb tube as the x-ray tube. In x-ray radiators constructed according to this principle, the anode of the x-ray tube is in direct contact with the liquid coolant that is present at the primary side of the heat exchanger, efficient cooling of the anode thereby being assured. Following the end of the examination of a patient, the anode of a rotating bulb tube, due to the direct contact with the liquid coolant, is already at essentially the same temperature as the liquid coolant after a relatively short time, for example approximately thirty seconds. Therefore, the anode of the rotating bulb tube differing from a rotating anode tube whose anode is cooled by convection does not have to function as a heat store. This function is assumed instead by the liquid coolant located in the primary circulation loop, this generally being oil, particularly insulating oil.
The anode of the rotating bulb tube at most serves as brief-duration heat store.
In order to assure an adequate heat capacity, it can be provided in a version of the invention the primary circulation loop contains an additional coolant storage tank mounted on the gantry.
A fast coupling, implemented preferably as a fast magnetic coupling having at least one electromagnet, is provided, which in a defined rotary position of the gantry, connects lines connected to the secondary side of the heat exchanger and arranged on the gantry for the coolant, to lines coming from the source and leading to the sink that are attached to the gantry housing. Whenever, after the implementation of an examination, the x-ray tube of the x-ray radiator and the rotation of the gantry are deactivated, the gantry is automatically moved into the defined position wherein, as a result of the fast coupling, a connection of the source that is stationary relative to the gantry and the sink of the liquid coolant that is stationary relative to the gantry ensues to the heat exchanger which rotates together with the gantry, so that the thermal energy stored therein that has arisen during the examination can be taken from the liquid coolant within a short time span. As a result, practically no standstill times of the computed tomography system occur, since the time required for removing a patient and for positioning the next patient on the displaceable bed is usually already adequate in order to achieve a cooling of the coolant serving as intermediate heat store.
According to one version of the invention, a coolant displacer that is preferably operated by compressed air is provided for emptying the secondary side of the heat exchanger and the lines for the liquid coolant that are connected thereto and arranged on the gantry. After the coolant has cooled, the flow of the liquid coolant is initially interrupted at the secondary side of the heat exchanger and any coolant which may be present in the secondary side of the heat exchanger and in lines connected thereto is removed, so that substantially no coolant is present at the secondary side of the heat exchanger during a subsequent examination. Such coolant, if present, could cause malfunctions of the computed tomography system, for example due to leakage from the secondary side.